ha! that´s a cool glitch though! You need to clear on each call. I’m not sure, but try setting the background color each time you call grab screen. if that doesn´t work call instead ofClear(0, 255) or whichever color you want to use to clear. if all fails, just draw a black rectangle as the background, that covers all the screen.
hope this helps.
Another option is using an ofFbo. it does pretty much the same internally than grabScreen, but allows you more control over the size of the screen. check the fbo examples and search the forum, there´s a lot about it. best!